Problem: Solve for $x$ and $y$ using substitution. ${5x-4y = -9}$ ${y = -x-9}$
Solution: Since $y$ has already been solved for, substitute $-x-9$ for $y$ in the first equation. ${5x - 4}{(-x-9)}{= -9}$ Simplify and solve for $x$ $5x+4x + 36 = -9$ $9x+36 = -9$ $9x+36{-36} = -9{-36}$ $9x = -45$ $\dfrac{9x}{{9}} = \dfrac{-45}{{9}}$ ${x = -5}$ Now that you know ${x = -5}$ , plug it back into $\thinspace {y = -x-9}\thinspace$ to find $y$ ${y = -}{(-5)}{ - 9}$ $y = 5 - 9$ $y = -4$ You can also plug ${x = -5}$ into $\thinspace {5x-4y = -9}\thinspace$ and get the same answer for $y$ : ${5}{(-5)}{ - 4y = -9}$ ${y = -4}$